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kernel: make sure procalarm() remaining time doesnt become negative

--- a/sys/src/9/port/alarm.c

+++ b/sys/src/9/port/alarm.c

@@ -63,14 +63,20 @@

 	Proc **l, *f;

 	ulong when, old;

+	when = MACHP(0)->ticks;

 	old = up->alarm;

-	if(old)

-		old = tk2ms(old - MACHP(0)->ticks);

+	if(old) {

+		old -= when;

+		if((long)old > 0)

+			old = tk2ms(old);

+		else

+			old = 0;

+	}

 	if(time == 0) {

 		up->alarm = 0;

 		return old;

 	}

-	when = ms2tk(time)+MACHP(0)->ticks;

+	when += ms2tk(time);

 	if(when == 0)

 		when = 1;

-- 

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